fix(github): match rel_X_Y_Z release tags

SQLAlchemy and Alembic tag releases as rel_2_1_5, which no tag candidate
matched, so their GitHub release notes were never found.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: Enes Yağız

src/backend/PkgDiff.Infrastructure/Services/GitHubScraperService.cs

@@ -336,6 +336,7 @@ private static IEnumerable<string> TagCandidates(string? packageId, string versi
         }
         yield return $"release-{version}";
         yield return $"release/{version}";
+        yield return $"rel_{version.Replace('.', '_')}";
     }
 
     [System.Text.RegularExpressions.GeneratedRegex(@"^[vV]?\d")]